New groenlandaspidid arthrodire from Antarctica 
39 
curved, meeting the ventral lamina along a 
prominent ventrolateral ridge. This ridge is only 
present in the posterior half of the plate. A less 
prominent, but still well-defined ridge is developed 
from the anterodorsal corner of the lateral lamina 
running posteroventrally towards the ventrolateral 
ridge, but tapering out well before reaching it. A 
small section of the overlap area for the AL plate is 
preserved, indicating that an extensive overlap was 
developed between the two plates. The ornament 
of the PVL consists of very fine, small tubercles for 
most of the plate, with larger, coarse tubercles 
present on the posterior half of the ventral lamina. 
The PVL differs from that of Croenlandaspis and 
Tiaraspis in its narrower ventral lamina and 
presence of a weak ridge running posteroventrally 
from the anterior corner of the lateral lamina. 
Spinal plate 
The Sp plate is almost completely preserved, 
missing only a section of the anterior region. It 
shows small pointed tubercles along the lateral 
margins and a row of slightly larger, recurved 
Figure 6 Comparison of right PDL and PL plates of A, 
Phlyctaenius stenosus (after Young 1983); B, 
Boomeraspis goujeti gen. et sp. nov.; C, 
Tiaraspis subtilis (modified after Gross 1962, 
and Schultze 1984), and D, Croenlandaspis 
antarcticus (after Ritchie 1975). Not to scale. 
tubercles lining part of the the mesial edge that 
protected the lateral edge of the pectoral fin. The 
bone is absent from most of the surface of the Sp, 
but impressions of the ornament show rows of 
aligned tubercles covered the external surface. In 
general form it is quite similar to the Sp in 
Groenlandaspis (Ritchie 1975). 
PHYLOGENETIC POSITION OF 
BOOMERASPIS 
Boomeraspis is placed within the family 
Groenlandaspididae because of its deep PDL plate 
which has a strongly inflected main lateral-line 
canal groove and a well-rounded, convex dorsal 
margin (Ritchie 1975, Goujet 1984). Although the 
inner surface of the PDL is unknown, the cross- 
sectional shape of the external surface is very 
similar to that of Groenlandaspis and suggests that it 
is likely that the two PDLs either met in dorsal 
contact or were closely situated under a high MD 
plate. Within the Groenlandaspididae only three 
genera are currently known, although others have 
been recognised and are currently being studied 
from the Early-Middle Devonian Mulga Downs 
Group of New South Wales by Dr Alex Ritchie 
(Ritchie 1975, p.571 "a new undescribed form from 
the Mulga Downs Group"). Clearly to resolve the 
question of whether Boomeraspis is more closely 
related to Groenlandaspis or to Tiaraspis depends 
largely on finding more complete remains, 
especially the head-shield. 
The primitive condition in all phlyctaeniids, 
based on the Early Devonian species from 
Spitsbergen and Canada (Goujet 1984, Young 
1983), is to have an elongate trunk-shield which 
has very long PL and PDL plates, and the PDL 
carries a straight lateral-line canal groove. The PL 
plate in most primitive phlyctaeniids may have a 
well-developed lateral ridge present (e.g., 
Phlyctaenius; Young 1983; arctolepids, arctaspids; 
Goujet 1984). Comparisons of the trunk-shield 
morphology would suggest that Boomeraspis, by 
having relatively longer PDL and PL plates, and a 
ridge on the lateral lamina of the PL plate, is more 
plesiomorphic than either Tiaraspis or 
Groenlandaspis. Groenlandaspis is considered more 
derived than Tiaraspis and Boomeraspis in having a 
PL plate that lacks any lateral ridge and has a well- 
developed, posteriorly-directed overlap surface for 
the PVL plate. The only synapomorphy that seems 
to unite Boomeraspis with either Tiaraspis or 
Groenlandaspis is the shape of the dorsal margin of 
the PDL plate, which in Groenlandaspis, and 
apparently (assumed in Boomeraspis) meets the 
opposing PDL underneath the MD plate. Both 
Ritchie (1975) and Schultze (1984) used this feature 
as an autapomorphy of the genus Groenlandaspis, 
so its suggested presence in Boomeraspis, based on 
